1. Technical Field
The invention relates to a stamp having a nanoscale structure and its use in a light-emitting element.
2. Reference to Related Application
This application claims the right of priority based on TW application Ser. No. 96129925, filed on Aug. 13, 2007, entitled “STAMP HAVING NANOSCALE STRUCTURE AND APPLICATIONS THEREOF IN LIGHT-EMITTING DEVICE”, and the contents of which are incorporated herein by reference.
3. Description of the Related Art
The light efficiency of solid-state lighting elements such as light-emitting diodes (LEDs) can be improved by increasing the internal efficiency and the light extraction, including the improvement of the efficiency of p-n junction in light-emitting layer, the use of different substrates and various epitaxial growth methods, etc. It can also be further improved by surface roughened processes. In an originally smooth surface, the light with the incident angle within 17 degree can be extracted, but it can be reflected by total reflection when the incident angle over 17 degree. The total reflection rate will be reduced and the light extraction will be increased if the surface was roughened. The most effective way is forming micro-structures on the surface of the chips or substrates to roughen interfaces or reduce the difference of interface refractive index to reduce the total reflection rate of the light extracted from the light-emitting diode chips. The roughened surface having nanoscale can be formed by the processes of the reactive ion etching (RIE) process or the inductively coupled plasma reactive ion etching (ICP-RIE) process, but the processes have poor uniformity and repetition. As the semiconductor industry develops and the processes innovate, a nanoscale process with high uniformity is no more out of reach. And now it is the research focus on the improvement of the light efficiency of light-emitting diodes in next century.
There are many different nano-technologies under development. In conventional semiconductor processes, the exposure light sources in photolithography processes are shifted from deep ultraviolet KrF 248 nm to ArF 193 nm, F2 157 nm, etc. The processes are belonged to optical lithography filed. For other processes does not belonged to the optical lithography filed such as E-beam direct write, SCALPEL, X-ray lithography, focused ion beam (FIB), etc., the processes have the abilities to limit linewidth to 100 nm, but the equipments costs are also multiply increased. Generally speaking, deep ultraviolet and SCALPEL have the abilities for mass production, but the equipments costs are very high. The E-beam direct write has extremely short wavelength (electron has extremely short wavelength), excellent DPI (can be less than 10 nm) and no need of mask, but it cannot be adopted in mass production like optical stepper can so its development is limited. The nano-imprint (NI) technology was presented by Professor S. Y. Chou in Princeton University in 1996. FIGS. 1A-1C show a process flow of nano-imprint technology. FIG. 1A shows an precise stamp 4 having a nanoscale pattern on the surface, and a substrate 6 coating with thermoplastic polymer material 5 such as PR is also provided. FIG. 1B shows the precise stamp imprinting process. By raising temperature to glass transition temperature, the thermoplastic polymer material 5 on the substrate 6 is imprinted by the precise stamp 4 and form patterns according to the pattern on the surface of the stamp. FIG. 1C shows the pattern forming process. After cooling, the thermoplastic polymer material 5 is solidified, the stamp is removed, the remained mask is cleaned by dry etching, and the pattern on the stamp is imprinted to the substrate 6. The process is similar with conventional hot embossing process. By using stamp having a nanoscale linewidth structure, imprint process of the nano-imprint (NI) technology can be proceeded in big scale and having the advantage in mass production to cover the disadvantage of low yield in E-beam direct write. The equipments cost is also far less than that of an optical stepper.
The nano-imprint (NI) technology can be defined as follows: imprint pattern from stamp having a nanoscale structures to specific materials in various way such as hot press, UV, light exposure to reach the goal of imprinting in big scale or mass production. Basing on the reason of developing this technology, it is obvious that the advantage of this technology is reaching a linewidth as small as in nanoscale, imprinting with high speed, and has advantages in mass production comparing to the present nano molding technologies. Therefore the point of this technology is forming stamps accurately. Generally, the stamps can be formed by electron-bean micro-lithography direct write process or ion optical lithography process and so on. But the cost is high because of time-consuming. Besides, most stamps have step-profile with rise and fall pattern. Although it can be used to control the light extraction of light-emitting diodes by, for example, forming photonic crystals, but the improvement in light extraction efficiency of light-emitting diodes is limited because most of the lights are extracted from the side wall of light-emitting diodes.